1. Technical Field
The present disclosure generally relates to the electronics field. More in particular, the present disclosure concerns a printed circuit board with reduced emission of electro-magnetic radiation.
2. Discussion of the Related Art
A printed circuit board (PCB) has the function of electrically connecting electronic components to each other, such as integrated circuits and power supplies of voltage or current.
It is necessary to use decoupling capacitors interposed between a power supply and an integrated circuit in order to provide the current necessary to perform abrupt transitions of the values of the supply current with high frequency components and in order to reduce the propagation of the noise generated by the integrated circuit.
In fact the power supply, the integrated circuit and the ground path form a loop which carries a return current towards ground with high frequency components, wherein said loop is a transmitting antenna supplied with the noise generated by the switching of the current absorbed by the integrated circuit, which can cause the emission of electro-magnetic radiation. Moreover, even the metal tracks of the printed circuit board can propagate the noise generated by the switching of the absorbed current and this noise is conducted on the wiring connected to the printed circuit board. Said wiring acts as potentially effective transmitting antennas, which further increase the emission of electro-magnetic radiation from the printed circuit board.
A known technique to perform the filtering of the noise generated by the switching of the current absorbed by the integrated circuits is to mount all the electronic components on the same outer side of the printed circuit board (indicated with “component side layer”) and to use one or more capacitors (also referred to as “decoupling capacitors” or “by-pass”) connected between the power supply voltage and the reference voltage to ground and placed close to the power supply and ground pins of the integrated circuit. The decoupling capacitors are connected between a supply pin and a ground pin of the integrated circuit: in this way each decoupling capacitor creates a local loop that carries a return current towards ground, wherein the local loop has a small size and thus it has a small inductance value. This allows to reduce the emission of electro-magnetic radiation of the board because the return current towards ground with high frequency components is limited in a loop having a small size and thus having a small inductance value, which is able to dampen the peaks of the dynamic fluctuations of the value of the power supply voltage caused by the abrupt transitions of the supply current and thus the emission of electro-magnetic radiation from the board is reduced.
Therefore the decoupling capacitor has the function of local reserve of electric charge, able to supply the current required during the abrupt transitions of the values of the supply current required for the operation of the integrated circuit supplied by supply current, dampening the peaks of the dynamic fluctuations of the power supply voltage caused by the abrupt transitions of the supply current.
Modern integrated circuits in many cases operate using two or more voltage power supplies having different values. For example, an integrated circuit comprises a supply pin for receiving a voltage of higher value (for example, 3.3 V or 5 V) to supply the interface circuits towards the outside connected with the Input/Output pins of the integrated circuit and comprises a further supply pin for receiving a voltage of lower value (for example, 1.2 or 1.8 V) to supply the logic circuits inside the integrated circuit. Moreover, the current flowing through the pin having the highest voltage has a trend of relatively low frequency components, while the current flowing through the pin with the lowest voltage has a trend of higher frequency components.
In an integrated circuit with two or more power supplies, it is necessary to electrically separate the return path of the current with high frequency components relating to a power supply voltage from the return path of the current with low frequency components relating to another power supply voltage, thereby preventing the noise generated by the switching of the current with high frequency transients from propagating within the board (for example, on the ground or power supply layers); this propagation may contribute to the generation of common mode currents that are the cause of considerable emission of electro-magnetic radiation by the board on which the integrated circuit is mounted.
The electrical separation between the return currents relating to two power supply voltages is typically obtained at the board level by keeping physically separated:                the return path towards ground of the electronic devices supplied by the power supply voltage and with supply current having high frequency components;        from the return path towards ground of the electronic devices supplied by the power supply voltage and with supply current having low frequency components.        
Moreover, it is also maintained within the integrated circuit the physical and thus electrical separation between the ground of the elements supplied by a power supply voltage from the ground of the elements supplied by the other power supply voltage: consequently, the integrated circuit comprises several ground pins, each corresponding to a respective power supply voltage, facilitating the star connection in order to isolate from each other the return paths relating to different power supplies.
The integrated circuits enclosed in a package of QFP (Quad-Flat Pack) type have connection pins towards the outside which are placed only around the perimeter of the package.
Moreover, QFP packages having a single ground exposed pad are available and this allows to have a greater number of pins available for other functionalities along the perimeter of the package and a better thermal dissipation. Said ground exposed pad is typically placed at the center of the top or bottom surface of the package and utilizes a part of this surface.
Printed circuit boards are known that mount the electronic components on both the external layers of the board, for example in the case wherein there is an integrated circuit of considerable size (for example, a micro-processor) which requires a high number of other electronic components. In this case the electronic components are mounted on an outer layer (indicated as “component side layer”) of the printed circuit board, while the integrated circuit is mounted on the other external layer (indicated as “soldering side layer”). In the case wherein the package of the integrated circuit mounted on the soldering side layer is of QFP type with a single ground exposed pad, said ground exposed pad is soldered on the soldering side and it is also necessary to connect the ground exposed pad with the components side layer by means of one or more via, in order to aid in the thermal dissipation of the QFP package.
The Applicant has observed that the connection of the ground exposed pad of the QFP package (mounted on the soldering side layer) with the components side layer creates a return loop towards ground that has a considerable size, because it crosses all the internal layers of the printed circuit board comprised between the soldering side layer and the component side layer (for example, in a board comprising four layers there are two inner layers, one for the connection of the signal pins and one for the ground): the considerable sizes generate a high inductance of the return loop towards ground that is created and generate a radiating structure internal to the board, thereby increasing the emission of electro-magnetic radiation of the printed circuit board towards the outside and towards the board itself. This may not allow conformance to electro-magnetic compatibility tests as defined in international standards CISPR25 and IEC61967.
Moreover, the Applicant has observed that the QFP packages with a single ground exposed pad form a return path towards ground common to two or more power supplies of the integrated circuit, because there is only one ground pin to which the return paths towards ground relating to two or more power supplies of the integrated circuit are connected; therefore it is not possible to maintain the separation between the return currents towards ground of two or more different power supply domains. Therefore, when the integrated circuit with QFP package with a single ground exposed pad and with at least two power supplies is mounted on a printed circuit board, common mode currents are created, which considerably increase the emission of electro-magnetic radiation from the board and thus do not allow the conformance to the electro-magnetic compatibility test.